The present invention relates to a cathode ray tube having a shadow mask, which is used for a television receiver, a computer display, and the like.
FIG. 6 is a cross-sectional view showing one example of a conventional color cathode ray tube. The color cathode ray tube 1 shown in FIG. 6 includes a substantially rectangular-shaped face panel 2 having a phosphor screen on its inner face, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 contained in a neck portion 3a of the funnel 3, a shadow mask 6 facing a phosphor screen 2a inside the face panel 2, and a mask frame 7 for fixing the shadow mask 6. Furthermore, in order to deflect and scan electron beams, a deflection yoke 5 is provided on the outer periphery of the funnel 3.
The shadow mask 6 plays a role of selecting colors with respect to three electron beams emitted from the electron gun 4. xe2x80x9cAxe2x80x9d shows a track of the electron beams. The shadow mask 6 has a flat plate provided with a number of substantially slot-shaped apertures formed by etching. The slot-shaped aperture is a through aperture through which electron beams pass.
In a color cathode ray tube, due to the thermal expansion caused by the impact of the emitted electron beams, the electron beam through aperture is shifted. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the electron beam through apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask is applied in advance, and then the shadow mask is stretched and held to the mask frame. When the shadow mask is stretched and held as mentioned above, even if the temperature of the shadow mask is raised, it is possible to reduce the amount of displacement between an aperture of the shadow mask and phosphor stripes of the phosphor screen.
FIG. 7 is a plan view showing an example of a shadow mask to which a tension force is applied mainly in the vertical direction of the screen. In FIG. 7, the direction indicated by arrow x is the horizontal direction of the screen, and the direction indicated by arrow y is the vertical direction of the screen. Apertures 8 are formed at constant pitches. Reference numeral 9 is referred to as a bridge, which is a portion between respective apertures 8. The bridge width has an effect on the mechanical strength of the shadow mask. More specifically, the bridge with a narrow width has a weak tension force particularly in the horizontal direction. If the bridge width is increased in order to improve the mechanical strength, the open area of the aperture is reduced, thus deteriorating the luminance intensity.
As mentioned above, the bridge width is related to the mechanical strength and the luminance intensity of the shadow mask, whereas the vertical pitch of the bridge is related to the doming amount of the shadow mask. The shadow mask is stretched mainly in the vertical direction. Therefore, the thermal expansion in the vertical direction is absorbed by the tension force, while the thermal expansion in the horizontal direction is transmitted in the horizontal direction through the bridges.
FIG. 8 is a graph showing an example of the relationship between the vertical pitch of the bridge and the doming amount (an example of a cathode ray tube for a 25-inch television is shown). FIG. 8 shows that the doming amount can be reduced by increasing the vertical pitch of the bridge.
However, the conventional color cathode ray tube suffers from the following problem. The doming amount can be reduced by increasing the vertical pitch of the bridge. In this case, however, moire stripes easily occur, thus causing the deterioration of the image quality. The moire stripe means a mutual interference stripe between scanning lines (luminescent lines) of the electron beams arranged at constant intervals and the regular pattern of the electron beam through apertures of the shadow mask.
Furthermore, when the vertical pitch of the bridge is increased, the bridges themselves may appear as dots on the screen, or may be recognized as a pattern in which the bridges are piled up (a brick-like pattern).
On the contrary, when the vertical pitch of the bridge is reduced, moire stripes are suppressed sufficiently and the bridges themselves are not noticeable. However, due to the increase of the shielding area of the scanning lines, the luminance property deteriorates, and at the same time, the doming amount is increased. Namely, it was difficult to suppress the doming amount and the occurrence of moire stripes at the same time.
It is an object of the present invention to solve the conventional problems described above by providing a cathode ray tube including a shadow mask of a tension system in which protruding portions are formed in apertures to reduce the doming amount and to suppress the occurrence of moire stripes at the same time.
To achieve the above object, a first cathode ray tube of the present invention includes a shadow mask made of a flat plate provided with a number of apertures and bridges between the neighboring apertures arranged in the vertical direction, wherein protruding portions protruding from the both ends of the horizontal direction of the aperture to the inside of the aperture are formed in the shadow mask, and a tip of the protruding portion is wider than a base of the protruding portion. According to the cathode ray tube described above, by forming the protruding portions, the vertical pitch of the bridge is maintained at a large value, while the occurrence of moire stripes can be suppressed in the same manner as the vertical pitch of the bridge is reduced. Furthermore, the tips of the pair of the protruding portions facing each other are formed separately, so that the thermal expansion in the horizontal direction is not transmitted between the protruding portions, and the doming can be prevented. In other words, it is possible to reduce the doming amount and also to suppress the occurrence of moire stripes at the same time. In addition, since the tip of the protruding portion is wider than the base, it is possible to suppress the deterioration of luminance while securing the shielding effect against the electron beams.
In the first cathode ray tube, it is preferable that the width of the protruding portion gradually increases from the base to the tip.
Furthermore, it is preferable that the tip of the protruding portion is extending more in the vertical direction than the base.
Next, a second cathode ray tube of the present invention includes a shadow mask made of a flat plate provided with a number of apertures and bridges between the neighboring apertures arranged in the vertical direction, and an electron beam passes through the apertures, wherein protruding portions protruding from the both ends of the horizontal direction of the aperture to the inside of the aperture are formed in the shadow mask, and a horizontal diameter of the electron beam passing through the aperture is smaller than a shortest distance in the horizontal direction between the portions facing each other via the aperture in the area where the protruding portions are formed at least in the vicinity of both edges of the shadow mask in the horizontal direction. According to the cathode ray tube described above, by forming the protruding portions, it is possible to reduce the doming amount and also to suppress the occurrence of moire stripes at the same time. Moreover, the cathode ray tube of the present invention also has improved effects to suppress the occurrence of moire stripes.
In the second cathode ray tube, it is preferable that the portions facing each other via the aperture in the area where the protruding portions are formed have horizontal cross sections asymmetrical to the center line of the aperture that is perpendicular to the surface of the shadow mask. According to the aforementioned cathode ray tube, the protruding portions are formed asymmetrically, so that the effects to suppress the occurrence of moire stripes can be improved even more.
Furthermore, it is preferable that the protruding portions are protruding from the both ends of the horizontal direction of the aperture to the inside of the aperture, and that tips of the protruding portions are facing each other.
Furthermore, it is preferable that one of the portions of the protruding portions facing each other via the aperture is an external wall portion positioned on the outer side of the shadow mask having a back inclining portion in the horizontal cross section inclining from a tip portion toward the outer side of the shadow mask as it approaches the back side of the shadow mask, and that the tip portion is positioned closer to the front side of the shadow mask than the center of the shadow mask in the thickness direction.
According to the aforementioned cathode ray tube, the position of the tip portion is shifted closer to the front side of the shadow mask, so that the effects to suppress the occurrence of moire stripes can be improved even more.
Furthermore, it is preferable that one the portions of the protruding portions facing each other via the aperture is an internal wall portion positioned on the inner side of the shadow mask having a front inclining portion in the horizontal cross section inclining from a tip portion toward the inner side of the shadow mask as it approaches the front side of the shadow mask, and that the tip portion of the external wall portion is positioned closer to the front side of the shadow mask than the tip portion of the internal wall portion.
Furthermore, it is preferable that the following inequality is satisfied, where an incident angle (xc2x0) of an electron beam is xcex1 (xcex1 greater than 0) in reference to the line vertical to the surface of the shadow mask; a shortest distance (mm) in the horizontal direction between the portions facing each other via the aperture of the protruding portions is SB; a displacement (mm) between the tip portion of the external wall portion and the tip portion of the internal wall portion in the thickness direction of the shadow mask is xcex94Z; and {1xe2x88x92(a horizontal diameter (mm) of the electron beam passing through the aperture)/(the shortest distance (mm) in the horizontal direction between the portions facing each other via the aperture)}xc3x97100 is a shielding rate B (%):
sin{90xc2x0xe2x88x92xcex1xe2x88x92(tanxe2x88x921xcex94Z/SB)}xc3x97(SB2+xcex94Z2)xc2xdxe2x89xa6(1xe2x88x92B/100)xc3x97SB.